The invention is directed to a particle beam apparatus and, in particular, to a particle beam apparatus having a particle beam generator, a first lens system for focusing the particle beam and a second lens system for imaging onto a specimen an intermediate image produced by the first lens system.
High resolution scanning electron microscopes have become necessary instruments in all areas of development and manufacture of microelectronic and optoelectronic components in order to visually evaluate sub-micrometer structures, in order to identify deviations from rated patterns and in order to acquire and evaluate topographical parameters such as heights, widths or angles of inclination. Conventional scanning electron microscopes, however, only achieve the required spatial resolution of fractions or a micrometer down to a few nanometers for small working distances and high accelerating voltages above approximately 20 kV. At these distances and voltages resist structures and integrated circuits are damaged by the high-energy electrons and non-conductive or poorly conductive specimens are charged.
The development of low-aberration condenser and objective lenses provided the possibility of designing scanning electron microscopes and electron beam measuring equipment that achieve the spatial resolution required for high-precision measurements even for low beam energies in the range of approximately 0.5 through 3 keV. In the lenses disclosed, for example, from U.S. Pat. Nos. 4,785,176, 4,831,266, and 4,728,790 or in Microelectronic Electronic Engineering, Vol. 7, Nos. 2 through 4 (1987) pages 163 through 172, an electrostatic regarding field overlaid on the focusing magnetic field to considerably reduce the lens aberrations given the same working distance is utilized.
In addition to lens aberrations, what is referred to as the Boersch effect also limits the spatial resolution. U.S. Pat. Nos. 4,713,543 and 4,675,524 disclose that electrons in the beam generator can be accelerated to a high energy of, for example, E&gt;10 keV in order to decelerate them to the desired, low end energy of, for example, E&lt;1 keV only immediately above the specimen.
U.S. Pat. No. 4,896,036 discloses a scanning electron microscope wherein the aberrations of an electrostatic detector objective lens are reduced using a corrector composed of a number of multiple elements.
All of the cited solutions that have been proposed require a re-design of the electron-optical column in order to improve the spatial resolution or at least require a considerable modification of the SEM electronics. As a result these low-aberration but complex lens systems cannot be used in conventional apparatus.